This invention relates to an improvement in acoustic materials, and more particularly but not by way of limitation, to acoustic materials which have a perforated layer of first metal adhesively bonded to a porous fibrous layer of a second metal and where the maintaining of electrical isolation therebetween is critical.
In the manufacturing of sound attenuation or acoustic material it is desirable to bond perforated sheet material of a first metal such as aluminum to microporous fibrous material made from a second metal such as stainless steel or the like. A problem frequently encountered is galvanic action between the adjacent dissimilar metals which eventually may cause their destruction. This is generally called "dissimilar metal reaction".
Attempts have been made to provide this necessary electrical isolation and yet maintain the desired flow through resistance to sound through the perforations of the perforated sheet and the pores of the porous fibrous material. A common method is to carefully select the proper type adhesive so that it performs both functions. Another method is to "layer" the selected adhesive by partially curing a first layer, adding a second layer and so on. Both of these methods, while successful in isolating the layers of dissimilar metal to preclude galvanic action and providing a predetermined flow through resistance were found to be inadequate, at least partially, when cutting the combined composite structure to proper size or drilling through the combined layers of metal. Cutting or drilling caused either an instantaneous breakdown of the isolation between the layers or caused a breakdown later when the material was in actual use. The breakdown of the physical isolation and concomitant breakdown of the electrical isolation then results in unacceptable destructive galvanic action between the two metallic layers.
One method for maintaining isolation is now employed with some success. This method employs the use of strips of non-metallic cloth applied between the adjacent metal layers during their fabrication along with the layer or layers of adhesive. This method was successful in maintaining isolation between the metal layers where cutting and drilling was required. The result, however, has several drawbacks in practice, namely, the locations to be cut and drilled have to be predetermined during construction and therefore the material could not be used later where different cutting and drilling locations are desired after construction of the acoustic panel, also the porous fibrous metallic material has irregular surfaces where the cloth was located which causes turbulence to air flow thereacross when in use, and the application of the cloth was very labor intensive.